This invention relates to automatic irrigation systems, and specifically to a method and apparatus for communication between an irrigation controller and peripheral sensors that provide data to the irrigation controller to assist in the irrigation control process.
Electronic irrigation controllers are commonly used to automatically control irrigation of vegetation such as lawns and gardens in both commercial and residential applications. These systems generally consist of a clock with a mechanism built in for activating an irrigation system at a specified day and time and for a particular duration. Water is applied based on a set schedule, without regard to changing weather conditions. With this type of system it is common to see sprinklers operating during or after a rainstorm when irrigation is unnecessary, and valuable water is being wasted.
Some systems have been enhanced to use various sensors, most commonly soil moisture sensors, to assist in irrigation scheduling. One of the more common embodiments of this type of system uses a conventional timer to schedule irrigation cycles, but will “override” the timer and not allow it to turn on if a certain moisture level is detected in the soil. U.S. Pat. No. 5,148,985 to Bancroft discloses such a system which is applicable to the scope of this patent. When it rains or during cool weather this type of system will tend to water only when needed, thus conserving water.
One drawback of this type of system is that the sensors generally require additional wires to be run to connect them to the controller. The sensors are usually placed in the soil some distance from the controller, which substantially increases the amount of wire required for a system that uses sensors over one that does not. This difference becomes even more pronounced when upgrading a system that has a conventional timer already installed. In most cases this requires trenching in established lawn and garden areas, digging under sidewalks, drilling holes in sides of buildings, etc. to run wires for sensors which causes installation costs to be higher.
U.S. Pat. No. 5,813,606 to Ziff discloses a radio controlled sprinkler control system which overcomes the problems of additional wires required for sensor connection. However, a radio link creates a set of new problems. One of the most significant is the lack of an electrical power connection to the sensors, thus requiring that the sensors use batteries or some other self-powering means which makes this an undesirable choice for many applications.
Most electronically controlled irrigation systems use electric control valves to turn the flow of water on and off. The actuating mechanism contained within the valves generally consists of a wire wound iron core solenoid which is actuated by the magnetic field that is produced when an electric current is applied to the terminals. These solenoids are characterized by a high level of inductance, usually somewhere on the order of 70 to 100 millihenries, which can be used to advantage and provides a key element to the functionality of circuitry used in this invention. The valves are generally installed in the field, near to where moisture or other type sensors may be located. There must be means for an electrical connection between a controller and valves for the controller to initiate action within the valves, which is generally in the form of PVC insulated solid copper wires, which are often called valve control wires. These valve control wires usually contain alternating current when a valve is active, generally at a voltage of 24 VAC.
Using these valve control wires to provide electrical connection to sensors as well as valves, such that a sensor located in the vicinity of the valve could tie into the valve wires, could eliminating the need for separate wires to be run for the sensor and thus save tremendously on installation costs to upgrade existing irrigation systems. However, this presents some difficulties because the voltage to drive the valves is much higher than what is compatible with most types of sensors. Also the valve itself presents a variable load which would cause erroneous readings from most sensors if attached directly to the valve control wires. An additional difficulty is the fact that the irrigation controller usually contains a type of solid state device known as a triac to switch the voltage applied to the valves on and off. One characteristic of triacs is that a rapidly changing voltage applied to one of its terminals can cause the triac to become active or “fire” prematurely. A sensor communicating over a valve wire could cause this to happen if signals to or from the sensor are not carefully controlled. My invention provides a means to overcome these problems and limitations.
Some of the principles used in this invention are similar to those used in prior art for power line networking, such as disclosed in U.S. Pat. No. 6,252,755 to Willer. However, it is my belief that these principles have not been previously applied to the area of irrigation system peripheral communications, and further that this invention provides new and unexpected results of this technology which are not obvious to those not invoking the inventive faculty.